1. Field of the Invention
This invention relates generally to magnetic recording disk drives for use with patterned media, wherein each data bit is stored in a magnetically isolated island or island on the disk, and more particularly to such a disk drive with an improved clock for writing the data.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase the data density. In patterned media, the magnetic material on the disk is patterned into small isolated data islands or islands arranged in concentric data tracks. Each island contains a single magnetic “bit” and is separated from neighboring islands by a nonmagnetic region. This is in contrast to conventional continuous media wherein a single “bit” is composed of multiple weakly-coupled neighboring magnetic grains that form a single magnetic domain and the bits are physically adjacent to one another. Patterned-media disks may be longitudinal magnetic recording disks, wherein the magnetization directions are parallel to or in the plane of the recording layer, or perpendicular magnetic recording disks, wherein the magnetization directions are perpendicular to or out-of-the-plane of the recording layer. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the regions between the islands must be destroyed or substantially reduced so as to render these regions essentially nonmagnetic. Alternatively, the media may be fabricated so that that there is no magnetic material in the regions between the islands. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., “Patterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording?”, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, pp. 990-995.
In one type of patterned media, the data islands are elevated, spaced-apart pillars that extend above the disk substrate surface to define recesses or trenches on the substrate surface between the pillars. This type of patterned media is of interest because substrates with the pre-etched pattern of pillars and trenches can be produced with relatively low-cost, high volume processes such as lithography and nanoimprinting. The magnetic recording layer material is then deposited over the entire surface of the pre-etched substrate to cover both the ends of the pillars and the trenches. The trenches are recessed far enough from the read/write head to not adversely affect reading or writing. This type of patterned media is described by Moritz et al., “Patterned Media Made From Pre-Etched Wafers: A Promising Route Toward Ultrahigh-Density Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp. 1731-1736.
In conventional magnetic recording where the data bits are written on continuous media, there is no requirement to write to precise positions on the media since all of the media contains magnetic material. However, with patterned media, because the data islands are single-domain, the transitions between bits may occur only between islands. Since the magnetic transitions are restricted to predetermined locations governed by the locations of individual islands, it is necessary to synchronize the reversal of current in the write head with the passing of individual islands under the head. Also to assure that the write head is precisely aligned with the islands on the patterned media, the media must be patterned perfectly with a single accurate period, and the effective motor speed of the spindle supporting the disks must be highly stable so that the write clock is perfectly synchronized with the islands as they pass beneath the write head. U.S. Pat. No. 6,754,017 B2, assigned to the same assignee as this application, describes a magnetic recording disk drive with patterned media that uses a special pattern-sensor that senses the data islands before they pass beneath the write head and generates a write-clock signal.
The above-cited related pending application Ser. No. 12/209,089 and assigned to the same assignee as this application relates to a patterned-media disk drive with an optical system for clocking the write data. A radiation source, such as a laser diode, directs radiation through an optical waveguide to a near-field transducer that is maintained near the disk surface. A radiation detector detects radiation reflected back from the transducer and its output signal represents the variation in reflected radiation when the transducer is near an island or pillar and when the transducer is near a nonmagnetic space or recess. The radiation detector output signal controls the write clock signal that controls write pulses to the write head, so the frequency and phase of the write clock signal can be matched to the frequency and phase of the islands as the disk rotates. However, the difference in reflected radiation power from the transducer between an island and a recess can be relatively small, making it difficult to generate a reliable radiation detector output signal.
What is needed is a patterned-media magnetic recording disk drive that uses an optical system for controlling the write clock like in the above-cited pending application, but wherein there is a greater contrast in reflected power from the transducer between an island and a recess.